The present invention is directed to a pressure sensitive adhesive comprised of a polymerized acrylic or methacrylic acid ester backbone having grafted thereto pendant sulfur, nitrogen or oxygen-containing heterocyclic moieties.
In the past, the practical use of adhesives with electrical conductivity has generally been limited to non-pressure sensitive adhesives. Generally, the conductive fillers used to impart conductivity are impractical for most adhesive-based applications either because they are prohibitively expensive (such as gold and silver) or they possess oil absorption characteristics that adversely affect the physical properties of the pressure-sensitive adhesive (PSA).
Various methods are known to impart conductivity to adhesives, including doping or the addition of conductive fillers.
It is known to add conductive blacks (i.e., carbon) to normally non-conductive PSA's, with the electrical properties of the PSA improving with the degree of carbon loading. See, for example, U.S. Pat. No. 3,778,306 which provides for the presence of between about 20-43 percent by weight of carbon in a pressure sensitive adhesive. However, at loadings high enough to provide good conductivity, a reduction in physical performance of the PSA characteristically results, affecting physical properties such as tack, peel, and shear. In the PSA formulation disclosed in the patent, loss of tack due to the presence of large amounts of conductive carbon is compensated for by the addition of a tackifying or plasticizing agent to the formulation and milling the rubber to effect softness.
Loss of adhesive tack is also recognized as a potential problem with respect to electrically-conductive pressure sensitive adhesives in U.S. Pat. No. 2,670,306. In this patent, the use of powdered graphite is stated to be preferred over carbon black to minimize the degree of tack loss.
U.S. Pat. No. 3,475,213 discloses an electrically-conductive adhesive tape which comprises a pressure sensitive adhesive and electrically-conductive particles distributed as a monolayer in the adhesive layer. U.S. Pat. No. 2,808,352 discloses an electrically conductive adhesive tape wherein the adhesive base is impregnated with finely divided silver particles. U.S. Pat. No. 4,367,745 discloses a conformable electrically conductive composition composed of a plurality of deformable, non-polar microspheres around which are dispersed electrically conductive particles. U.S. Pat. No. 4,548,862 is directed to a flexible tape having bridges of electrically conductive particles extending through the adhesive layer. The particles have ferromagnetic cores which by magnetic attraction can serve to form the requisite bridges. U.S. Pat. No. 4,588,762 discloses a pressure sensitive adhesive composition consisting of both a viscoelastic polymeric adhesive phase and an electrically conductive aqueous phase containing a water receptive polymer, a humectant, and an electrolyte.
It would be desirable to provide a pressure sensitive adhesive which exhibits electrically-conductive properties without the need for the use of conductive fillers or for doping the adhesive with an electrically-conductive liquid phase.
It is known to provide electrically-conductive polymers by inclusion within the polymer of a monomer which serves to impart electrical conductivity to the polymer. Exemplary of such monomers are heterocyclic compounds containing such as oxygen, sulfur or nitrogen-containing monomers. In particular, thiophene-based monomers have been found to be particularly useful in imparting electrical conductivity to the polymer. See, for example, U.S. Pat. Nos. 4,900,782; 4,959,430; 5,035,926; 5,111,327; 5,331,056; 5,385,956; 5,430,073; 5,432,237; and EP 652,242. However, to date, no one has provided a pressure sensitive adhesive formed from a copolymer a portion of which enables the adhesive to exhibit satisfactory electrical conductivity.